Charles Moosbrugger, editor All rights reserved

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CHAPTER 1 Introduction to Magnesium Alloys

MAGNESIUM is the lightest common struc- application. On the other side of the ledger, the Designation Systemstural metal with a density of 1.74 g/cm3 in its strong galvanic potential of magnesium and itssolid state. The data in this collection focus on weak surface oxidation make corrosion behaviormechanical and physical properties of magne- a major consideration. Fortunately, good design No designation system has universal accep-sium that are relevant to engineers in the design practices and preventive measures are available tance. Names of alloys have evolved from tradeof lightweight components and structures. Other to ameliorate environmental degradation. names of the pioneering companies to chemicalreferences (Ref 1, 2) are suggested for details on Structural applications include automotive, and numerical systems.the various manufacturing processes employed. industrial, materials handling, commercial, and The ASTM Standard Alloy DesignationThis collection contains physical data that are aerospace equipment. The automotive applica- System is widely used by the industry. Details ofhelpful for the metal processor and for process tions include clutch and brake pedal support the ASTM system are given in Table 1 (Ref 2, 4).simulation. The effect that various manufactur- brackets, steering column lock housings, and As an example of how this alphanumeric systeming processes have on the resulting magnesium manual transmission housings. In industrial ma- works, consider magnesium alloy AZ91E-T6.components is evident by comparing the data chinery, such as textile and printing machines, The first part of the designation, AZ, signifiesamong the various casting and wrought forms. magnesium alloys are used for parts that oper- that aluminum and zinc are the two principal al- Proper use of the data requires a clear un- ate at high speeds and must be lightweight to loying elements. The second part, 91, gives thederstanding of the material behavior the values minimize inertial forces. Materials-handling rounded-off percentages of aluminum and zincrepresent. Consensus definitions of material equipment includes dockboards, grain shovels, (9 and 1, respectively). The third part, E, indi-properties are found in the Glossary of Terms in and gravity conveyors. Commercial applications cates that this is the fifth alloy standardized withthis book. include handheld tools, luggage, computer hous- approximately 9% Al and 1% Zn as the principal Effort has been made to attribute the source of ings, and ladders. Magnesium alloys are valu- alloying additions. Letters are used in alphabeticthe data. Multiple sources are given when avail- able for aerospace applications because they order, except for O and I, which are not used.able to give the reader an indication of the ve- are lightweight and exhibit good strength and The fourth part, T6, denotes that the alloy is so-racity and range of the data. The comparison of lution treated and artificially aged. The common stiffness at both room and elevated temperaturesdata found in literature is made challenging by tempers are listed in Table 1. (Ref 2).the variety of test methods and reporting formats Pure magnesium (98.8% Mg or higher) is Pyrotechnics. The first applications of mag-employed by researchers, and by varied designa- designated by the required minimum amount of nesium powder were components of fireworks,tions given to alloys. magnesium. Several grades are commercially flares, and other incendiary devices to produce available for metallurgical and chemical uses. brilliant white light. Fine magnesium wire was These are rarely used for structural engineering used for photographic flash bulbs. Magnesium is applications. The grades are designated 9880A still used in fire starters for survival kits. (UNS M19980) and 9880B (UNS M19981) forRepresentative Applications Metallurgical. Magnesium is used as an al- 98.80% min; 9990A (UNS M19990) for 99.90% loying element in nonferrous alloys, such as min, 9995 (UNS M19995) for 99.95% min, and Magnesium is used in a wide variety of appli- aluminum, zinc, and lead. It is used as an oxy- 9998A (M199980) for 99.98% min. The Unifiedcations from medical and metallurgical to chem- gen scavenger in nickel and copper alloys and Numbering System (UNS) is a complementaryical and pyrotechnic. Although the main focus as a desulfurizer in iron and steel production. designation system of ASTM and the Society ofof this book is on the structural applications of Magnesium improves the toughness and ductil- Automotive Engineers (SAE). It is not a specifi-magnesium, other uses of magnesium alloys are ity of cast iron by making the graphite particles cation because it does not establish requirementsalso addressed. nodular. This is the greatest use of magnesium such as mechanical properties or heat treatment, Structural. The high strength-to-weight ratio by weight. but it provides identifying numbers that are use-of magnesium alloys is usually a prime reason Electrochemical Applications. Magnesium ful for searching literature. All magnesium met-for considering these materials in engineering is highest on the electromotive series among als and alloys have UNS numbers starting withdesigns. High stiffness-to-weight, castability, metals in salt water, making it desirable as a M, but the M category is defined as “miscella-machinability, and excellent damping are desir- sacrificial anode for cathodic protection. Con- neous nonferrous metals and alloys,” so severalable properties of magnesium alloys that factor structive uses of this mechanism are employed UNS M alloy numbers are not magnesium-into the material selection process. The unique- in batteries. based.ness of the magnesium alloys is illustrated in Medical. Magnesium alloys are used in por- Using the ASTM alphanumeric designationan Ashby diagram of Young’s modulus against table medical equipment where light weight is system encourages grouping magnesium alloysdensity among engineering materials (Fig. 1, advantageous. It is also employed for wheel- by principal alloy composition:Ref 3). The position at a corner of the triangu- chairs used in sporting activities (where everylar shape representing all engineering alloys and ounce is critical). Because of magnesium’s bio- • Magnesium-manganese (M)its position shared by engineering composites compatibility and bioabsorbability, alloys with • Magnesium-aluminum-manganese (AM)highlight the special qualities of magnesium al- other biocompatible elements (such as calcium) • Magnesium-aluminum-zinc-manganeseloys. The thermal properties of magnesium fac- are being evaluated for cardiovascular stents and (AZ)tor into the castability of the alloys and serve in orthopedic devices for internal bone fixation. • Magnesium-zirconium (K)2 / Engineering Properties of Magnesium Alloys

Fig. 1 shby diagram of Young’s modulus, E, plotted against density, ρ, for various engineered materials. The heavy envelopes enclose data for a given class of material. The diagonal A contours show the longitudinal wave velocity. The guide lines of constant E/ρ, E1/2/ρ, and E1/3/ρ allow selection of materials for minimum weight, deflection-limited, design. Source: Ref 3

• Magnesium-zinc-zirconium (ZK), with rare The Physical Properties of the Alloys Are melting, casting, and welding alloys. It is suc- earth (ZE) Influenced by Their Chemical Composition. cessfully used in die-cast and wrought products• Magnesium–rare earth metal–zirconium In general the constituent elements have the fol- but must be used judiciously in sand-casting be- (EZ) lowing effects. cause it coarsens the grain.• Magnesium-silver–rare earth metal–zirco- Aluminum has a favorable effect on magne- Calcium is added in small amounts to help nium (QE) sium. It is used up to 10 wt%, with optimum metallurgical control because it increases grain• strength and ductility at approximately 6%. Alu- refinement. It is added just prior to pouring to Magnesium–yttrium rare earth metal–zirco- minum improves strength and hardness. It wid- reduce oxidation. It improves rolling of sheet nium (WE) ens the melting range, which makes the alloy products, where it is used below 0.3 wt% so• Magnesium-zinc-copper-manganese (ZC) easier to cast. With aluminum content higher the product can be welded without cracking. It• Magnesium-aluminum-silicon-manganese improves thermal and mechanical properties of than 6%, the alloy is heat treatable. (AS) Beryllium is used in small amounts (up to the alloy, including creep resistance. There is• Magnesium-aluminum-strontium (AJ) 0.001 wt%) to decrease surface oxidation when interest in magnesium-zinc-calcium alloys for Introduction to Magnesium Alloys / 3

First part (AZ) Second part (91) Third part (E) Fourth part (T6)Indicates the two principal alloying elements Indicates the amount of the two principal Distinguishes between different alloys with the Indicates condition (temper) alloying elements same percentages of the two principal alloying elementsConsists of two code letters representing the Consists of two numbers corresponding to Consists of a letter of the alphabet assigned in Consists of a letter followed by a numbertwo main alloying elements arranged in order rounded-off percentages of the two main order as compositions become standard (separated from the third part of theof decreasing percentage (or alphabetically if alloying elements and arranged in same order designation by a hyphen)percentages are equal) as alloy designations in first partA, aluminum Whole numbers Letters of alphabet except I and O F, as fabricated O, annealedB, bismuth(a) H, strain hardened (wrought products only) Subdivisions of H temper:C, copper H1, plus one or more digits, strain hardened only H2, plus one or more digits, strain hardened and partially annealedD, cadmium(a) H3, plus one or more digits, strain hardened and then stabilized W, solution heat treated, unstable temper, only for alloys that spontaneously age at room temperatureE, rare earth T, Thermally treated to produce stable tempers (other than O,H, and F) Subdivisions of T temper:F, iron(a) T1, cooled and naturally agedH, thorium T3, solution heat treated and cold workedJ, strontium T4, solution heat treatedK, zirconium T5, cooled and artificially aged only T6, solution heat treated and artificially agedL, lithium T7, solution heat treated and stabilized T8, solution heat treated, cold worked, and artificially agedM, manganese T9, solution heat treated, artificially aged and cold worked T10, cooled, artificially aged, and cold workedN, nickel(a)P, lead(a)Q, silverR, chromium(a)S, siliconT, tinV, gadoliniumW, yttriumY, antimony(a)Z, zinc(a) Elements found in Ref 2 that are not in ASTM Ref 4. Ref 4 notes that thorium, lithium and tin are listed for historical purpose. Source: Ref 2, 4

medical device applications because these three resistance, the upper limit of iron content is termetallic compounds that can be removed dur-elements are found naturally in the human body specified at 0.005 wt%. Commercial grade al- ing melting. Commercial alloys rarely containand are biocompatible and biodegradable. loys where corrosion is not a prime concern may over 1.5 wt% Mn; in the presence of aluminum Cerium improves corrosion resistance, in- contain iron as high as 0.01 to 0.03 wt%. the solubility of Mn is reduced to approximatelycreases plastic deformation capability such as Lithium is relatively soluble in magnesium, 0.3 wt%.elongation, increases work hardening rates, and so it has attracted interest for making ultra-light Nickel increases yield and ultimate strengthreduces yield strength. structural materials; lithium has a solid density at room temperature, but negatively affects Copper improves room temperature and high- of 0.53 g/cm3, 30% the weight of magnesium. ductility and corrosion resistance in even smalltemperature strength, but in quantities greater Lithium increases the ductility of magnesium amounts. Like iron, for commercial gradesthan 0.05 wt% it adversely affects corrosion re- alloys, thus improving formability, but it de- where corrosion is not a concern, Ni content cansistance and ductility. creases strength. average 0.01 to 0.03 wt%, but for maximum cor- Iron is one of the most harmful impurities as Manganese increases saltwater corrosion re- rosion resistance the upper limit of Ni content isit significantly reduces the corrosion resistance sistance of aluminum and aluminum-zinc alloys specified at 0.005 wt%.of magnesium alloys. For maximum corrosion by capturing iron and other heavy metals in in- Neodymium improves material strength.4 / Engineering Properties of Magnesium Alloys

Rare earth metals increase high-temperature

freezing range of the alloys, which reduces po- ASTM—ASTM International

rosity. They also reduce weld cracking. Rare B80-15 Magnesium-Alloy Sand Castings Chemical and tensile requirementsearths are added to alloys in the form of mis-chmetal or didymium. Mischmetal is a natural B90/B90M-15 Magnesium-Alloy Sheet and Plate Chemical and tensile requirementsmixture of 50 wt% cerium with the remainder (customary and metric units)being lanthanum and neodymium. Didymium B91-12 Magnesium-Alloy Forgings Chemical and tensile requirementsis a natural mixture of approximately 85% neo-dymium and 15% praseodymium. Check with B92/92M-11 Magnesium Ingot and Stick for Remelting Chemical composition for unalloyedindustry standards, such as ASTM, for the exact magnesium in ingot and stick for remelting, (customary and metric units)material specifications by product form. Silicon can increase molten alloy fluidity. B93/B93M-15 Magnesium Alloys in Ingot Form for Sand Chemical composition of alloys for remelt toIt is only used in high-pressure die-casting alloys. Casting, Permanent Mold Castings, and Die manufacture forms listed in title (customaryIt improves elevated temperature properties, es- Castings and metric units)pecially creep resistance. It decreases corrosion B94 -13 Magnesium-Alloy Die Castings Chemical and tensile requirements for highresistance if iron is also present in the alloy. pressure die castings Strontium is used in conjunction with other B107/B107M-13 Magnesium-Alloy Extruded Bars, Rods, Chemical and tensile requirementselements to enhance creep performance. Profiles, Tubes, and Wire (customary and metric units) Silver improves mechanical properties by in- B199-12 Magnesium-Alloy Permanent Mold Castings Chemical and tensile requirementscreasing the response to age hardening. Thorium was used to increase creep strength B403 -12 Magnesium-Alloy Investment Castings Chemical and tensile requirementsat elevated temperatures. It improved weldabil-ity of alloys also containing zinc. It is no longer B843-13 Magnesium Alloy Anodes for Cathodic Chemical requirements for cast and extrudedused because of its radioactivity. Protection alloys used as anodes Tin is useful when used with small amounts B296-03(2014) Temper Designations of Magnesium Alloys, Practices forof aluminum to improve ductility, and it reduces Cast and Wroughtthe tendency to crack during processing, such as B661-12 Heat Treatment of Magnesium Alloys Practices forforging. It is not a major alloying element. Yttrium has relatively high solubility in B951-11 Codification of Unalloyed Magnesium and Practices formagnesium and enhances high-temperature Magnesium-Alloys, Cast and Wrought(up to 300 °C, 570 °F) strength and creep per- B953-13 Sampling Magnesium and Magnesium Practices forformance when combined with other rare earth Alloys for Mass Spectrochemical Analysismetals. Zinc is second to aluminum as the most ef- B954-15 Analysis of Magnesium and Magnesium Test method Alloys by Atomic Mass Spectrometryfective and commonly used alloying metal withmagnesium. In conjunction with Al, it increases ISO—International Organization for Standardization

room-temperature strength. Additions of 1 wt% 16220:2005 Magnesium and magnesium alloys— Chemical compositions and mechanicalor greater when Al is 7 to 10 wt% tend to make Magnesium alloy ingots and castings properties of separately cast samples and samples cut from castings. Last reviewedthe alloy prone to hot cracking. Zinc increases 2015alloy fluidity in casting. When added to magne-sium alloys with nickel and iron impurities, it can 3116:2007 Magnesium and magnesium alloys— Chemical compositions and mechanicalimprove corrosion resistance. In combination Wrought magnesium alloys properties. Last reviewed 2013with Zr and rare earth metals, it produces pre- 8287:2013 Magnesium and magnesium alloys— Chemical compositionscipitation-hardenable alloys with good strength. Unalloyed magnesium—Chemical Zirconium has a powerful grain-refining ef- compositionfect in sand and gravity castings. Zirconium isadded to alloys containing zinc and rare earth 26202:2007 Magnesium and magnesium alloys— Chemical compositions Magnesium alloys for cast anodesmetals (not combined with alloys containing CEN—European Committee for Standardizationaluminum or manganese) when it serves as agrain refiner (Ref 1). EN 1753:1997 Magnesium and magnesium alloys— Chemical and tensile requirements for alloys International and Commercial Desig- Magnesium alloy ingots and castingsnations and Standards. Industrial and gov-ernment standards are convenient means of EN 12438:1998 Magnesium and magnesium alloys— Chemical compositions and test method to Magnesium alloys for cast anodes determine potential of anodesensuring consistent performance of the materialand identifying alloys. Standards include man-datory requirements and may include nonman-datory typical values and information. See Table ISO Standard 16220:2005, Magnesium mechanical properties for wrought magnesium2 for the major ASTM and International Stan- and magnesium alloys—magnesium alloy in- (Ref 7). There are also ISO standards for unal-dards devoted to magnesium. gots and castings, last reviewed in 2015, pro- loyed magnesium (ISO 8287:2013, Ref 8) and A comparison of the designation of magne- vides chemical compositions of magnesium magnesium used for anodes (ISO 26202:2007,sium alloys used by the various organizations is alloy castings and mechanical properties of Ref 9).found in Table 3 (Ref 5). This also has several separately cast samples and samples cut from EN 1753:1997, Magnesium and magnesiumdesignations (British Standards, BS) that may be castings (Ref 6). A new version is under de- alloys—magnesium alloy ingots and castings,useful for interpreting older technical literature velopment. ISO 3116:2007, last reviewed in provides similar information from CEN, theand test results. 2013, provides chemical compositions and European Committee for Standarization (Ref 5). Introduction to Magnesium Alloys / 5

Available Product Forms Table 3 Similar magnesium alloy designations

EN Standard The thermal properties of magnesium alloys ASTM Symbol Number ISO BS Designation Other previouspromotes cost-effective casting. A majority ofthe alloys are created for casting processes. Most AZ81 EN-MCMgAl8Zn EN-MC21110 Mg-Al8Zn1 MAG 1 A8are amenable to sand, permanent mold, and in- AZ91 EN-MCMgAl9Zn1(A) EN-MC21120 Mg-Al9Zn, No1 MAG 7 Cvestment casting. A smaller number are best for AM60 EN-MCMgAl6Mn EN-MC21230 … … …high-pressure die-casting, which is the most usedcasting process. Together, magnesium alloys are AS41 EN-MCMgAlSi EN-MC21320 … … …the third most popular nonferrous casting mate- EQ21 EN-MCMgRE2Ag1Zr EN-MC65220 … MAG 13 …rial, behind aluminum and copper-based alloys. EZ33 EN-MCMgRE3Zn2Zr EN-MC65120 Mg-RE32Zr MAG 6 ZRE1 Another subset of magnesium alloys are de-signed for the wrought products such as wire, QE22 EN-MCMgRE2Ag2Zr EN-MC65210 Mg-Ag3REZr MAG 12 MSRrod, hollow tubes, shapes, sheet and plate, and WE43 EN-MCMgY4RE3Zr EN-MC95320 … … …forgings. WE54 EN-MCMgY5RE4Zr EN-MC95310 … MAG 14 High-Pressure Die-Casting Alloys. The die- ZC63 EN-MCMgZn6Cu3Mn EN-MC32110 … …. …casting process is ideally suited to high-volumeproduction where the high cost of the die can be ZE41 EN-MCMgZn4RE1Zr EN-MC35110 Mg-Zn4REZr MAG 5 RZ5amortized by the large production volume. Mag- Source: Ref 5nesium alloys allow for high production ratesdue to their relatively low melting temperatures,thermal conductivity, and other factors. Tradi- Table 4 Nominal compositions of magnesium casting alloys for die castingtionally, material is injected into the die in liquidform, but the use of semisolid injection, thixo- Alloying elementsmolding, is increasing. Alloy UNS number Al Mn Si Sr Zn Re Mg Die-casting alloys are mainly of the Mg-Al-Zn type (AZ), for example, AZ91. Two versions AJ52A M17520 5 0.4(a) … 2 … … balof this alloy from which die castings have been AJ62A M17620 6 0.4(a) … 2.4 … … balmade for many years are AZ91A and AZ91B. AM50A M10500 5 0.35(a) … … … … balThe only difference between these two ver-sions is the higher allowable copper impurity in AM60A M10600 6 0.3 … … … … balAZ91B, which can be made from scrap magne- AM60B M10602 6 0.35(a) … … … … balsium. The AZ91D version is a high-purity ver- AS21A M10410 2.25 0.35 1 … … … balsion of the alloy in which the nickel, iron, andcopper impurity levels are very low and the AS21B M10412 2.25 0.1 1 … … … baliron-to-manganese ratio in the alloy is strictly AS41A M10410 4.25 0.35 1 … … … balcontrolled. This high-purity alloy shows amuch higher corrosion resistance than the ear- AS41B M10412 4.25 0.50(a) 1 … … … ballier grades and has good mechanical and physi- AZ91A M11910 9 0.13 min … … 0.7 … balcal properties. The nominal composition and AZ91B M11912 9 0.13 min … … 0.7 …properties of the die-casting alloys are given inTables 4 and 5, respectively. AZ91D M11916 9 0.30(a) … … 0.7 … bal When greater ductility is needed, the Mg- AE44 … 4 0.25 … … 4 balAl-Mn (AM) alloy is used. AM60B has greater (a) Manganese content is dependent on iron contaminant content. Source: Adapted from Ref 10toughness and more elongation than AZ91D,while retaining good corrosion resistance. The Mg-Al-Si-Mg (AS) alloys are used for Table 5 Summary of selected die cast propertieselevated temperatures (up to 175 °C, 350 °F)where superior creep strength is needed, while Alloy(a) General characteristicsretaining good corrosion resistance. More recent work has produced alloys such AZ91D Most commonly used die casting alloy; good strength at room temperature, good castability, good atmospheric stability, excellent saltwater corrosion resistanceas AJ52A and AJ62A containing strontium,with the aim of improving the high-temperature AM60B Good elongation and toughness, excellent saltwater corrosion resistance, good yield and tensile propertiesproperties with good corrosion resistance. AE44 AS21A,B Best creep resistance of die casting alloys, good room-temperature properties, useful in high-temperaturealloy, containing rare earth metals, has also been applicationsintroduced. This type of magnesium alloy is in- AS41A,B Good creep resistance up to 175 °C (350 °F), good room-temperature properties, excellent saltwater corrosioncreasingly being used in the automotive indus- resistance, useful in high-temperature applicationstry. Sand, Permanent Mold, and Investment (a) All alloys are in the as-cast condition. Source: Ref 2Casting. Several alloying systems are used forthese processes. In general, alloys that are nor- Nominal compositions of these cast alloys are They are not suitable for applications in whichmally sand cast are also suitable for permanent found in Table 6. General characteristics of the temperatures of over 95 °C (200 °F) are experi-mold casting. The exceptions to this are the Mg- cast alloys are in Table 7. enced. The Mg-RE-Zr alloys were developed toZn-Zr alloys (for example, ZK51 and ZK61A) The Mg-Al and Mg-Al-Zn alloys are generally overcome these limitations. A small amount ofthat exhibit strong hot-shortness tendencies easy to cast but are limited in certain respects. zirconium is a potent grain refiner. The two Mg-and are unsuitable for permanent mold casting. They exhibit microshrinkage when sand cast. Zn-Zr alloys originally developed, ZK51A and6 / Engineering Properties of Magnesium Alloys

nor zirconium. The alloy can be extruded at high controlled rate of one deformation is desirable Engineering Properties andrates and exhibits good strength properties. The because it facilitates control of the plastic flow Component Functionscorrosion resistance of ZC71 is similar to that of of metal; therefore, hydraulic press forging isAZ91C, but it falls short of that of AZ91E. the most commonly used process. Magnesium, Sheet and plate are rolled magnesium- which has a hexagonal crystal structure, is more This book presents a collection of magnesiumaluminum-zinc (AZ and photoengraving grade, easily worked at elevated temperatures. Conse- engineering properties data sheets to aid designPE) and magnesium-zinc-rare earth (ZE) (Ta- quently, forging stock (ingot or billet) is heated engineers in choosing from among the manyble 8). to a temperature between 350 and 500 °C (650 available alloys and product forms. To clarify AZ31B is the most widely used alloy for sheet and 950 °F) prior to forging. the importance of each property, their effect onand plate and is available in several grades and function must be understood. See also the Glos-tempers. It can be used at temperatures up to 100 sary of Terms in this book.°C (212 °F). Density. Magnesium with a density of ap- Alloy PE is a special-quality sheet with ex- Design Consideration proximately 1.74 g/cm3 at 20 °C (68 °F) is thecellent flatness, corrosion resistance, and etch- lightest of structural metals. The difference be-ability. It is used in photoengraving. ZE10A is tween alloys is slight, varying with composition.a newer grade that can we welded without the The enterprise of design involves many steps Currently, alloys with lithium as an alloying el-need of postweld stress relief. from getting clarity on customer requirements to ement are being developed to make ultra-light Good formability is an important requirement marketing the product. alloys. Light weight is desired for aerospace andfor most sheet materials. When correct tempera- Material selection is an integral part of the automotive applications and personal-use items.tures and forming conditions are employed, all process of design engineering. The properties The density must be considered when evaluatingmagnesium alloys can be deep drawn to about of materials as they relate to performance of the the cost per volume of material.equal reduction. finished product and the behavior of the material When the design environment requires that Forgings are made of AZ31B, AZ61A, during manufacturing processes must be consid- corrosion behavior be evaluated, it should be re-AZ80A, and ZK60A; the compositions and ered. membered that the density or the material affectsproperties of these alloys are listed under ex- Whether creating new materials or choosing the conversion of corrosion rates from weighttruded bars and shapes in Table 8. Alloy AZ31B from existing commercially available materials, loss to loss of thickness.may be used for hammer forgings (whereas the the material of a component will need certain Melting Range. The range between the soli-other alloy are almost always press forged). The characteristics to perform the component’s in- dus and liquidus points is given. Generally aAZ80A alloy has greater strength than AZ61A tended function. Each function or characteristic lower range lessens the energy required to meltand requires the slowest rate of deformation of will have a corresponding material property that the alloy for casting and increases speed of cast-the magnesium-aluminum-zinc alloys. ZK60A will be helpful in evaluating a material’s suit- ing. The maximum heat-treating temperaturehas essentially the same strength as AZ80A but ability to satisfy that particular need. should be noted as well.with greater ductility. To develop maximum Before choosing a material, it is customary to Service Temperatures. Recommended maxi-properties, both AZ80A and ZK60A are heat formalize the desired performance accompanying mum operating temperatures are often listed.treated to the artificially aged (T5) condition; the material needs through formal specifications: These are guides, but the real world is compli-AZ80A may be given the T6 solution heat treat- cated. The elevated temperature material prop-ment, followed by artificial aging to provide • Product or component specifications detail- erties should be consulted, and a determinationmaximum creep stability. ing the customer needs should be made to evaluate the need for elevated Hydraulic and mechanical processes are • Material specifications detailing the require- temperature testing of the alloy or components.both used for forging magnesium. A slow and ments and quality When evaluating published elevated data, note8 / Engineering Properties of Magnesium Alloys

whether the test was done at the elevated tem- on this stress-strain curve is the average longitu- tabular form, are the ultimate tensile strengthperature or at room temperature after exposure dinal stress in the tensile specimen. It is obtained (or just tensile strength), yield strength or yieldto a high temperature. by dividing the load, P, by the original area of point, percent elongation, and reduction in area. Specific Heat. This is the amount of heat the cross section of the specimen, A0: The first three are strength parameters; the lastenergy required to raise a unit mass 1 °C. Mag- two indicate ductility.nesium has a high specific heat, so for equal S = P/A0 (Eq 1) The general shape of the engineering stress-masses of material, magnesium will stay cooler strain curve (Fig. 2) requires further explana-as a heat sink. If comparing equal volumes of The strain, e, plotted on the engineering tion. This curve represents the full loading ofmetal, the lighter density of magnesium may stress-strain curve, is the average linear strain, a specimen from initial load to rupture. It is aresult in a heat sink rising in temperature more which is obtained by dividing the elongation of “full-range” curve. Often engineering curves arequickly. the gage length of the specimen, δ, by its origi- truncated past the 0.2% yield point (YS) or at the Thermal Conductivity. This is related to nal length, L0: maximum stress. This is the case of many of thedimensions rather than mass. Thermal conduc- curves in this book.tivity is of interest for casting design. There is e = δ/L0 = ΔL/L0 = (L − L0)/L0 (Eq 2) Segments of the curve are significant repre-often concern with regard to the flammability of sentations of material behavior.magnesium. Its specific heat and thermal con- Because both the stress and the strain are ob- Proportional limit. From the origin, 0, theductivity retard ignition in components of any tained by dividing the load and elongation by initial straight-line portion is the elastic region,tangible size. constant factors, the load-elongation curve has where stress is linearly proportional to strain. Electrical Conductivity and Resistivity. the same shape as the engineering stress-strain When the stress is removed, if the strain disap-Generally electrical properties are not prime curve. The two curves are frequently used inter- pears, the specimen is considered completelyconcerns, but they do play a role in corrosion. changeably. elastic. The point at which the curve departsElectrical conduction and resistance are impor- The units of stress are force/length squared, from the straight-line proportionality, A, is thetant for battery applications and corrosion. and the strain is unitless. The strain axis of proportional limit. Beyond this point permanent Strength. The ability of a component to resist curves in this book are given units of in./in. or deformation occurs.failure by yielding or fracture is called strength. mm/mm for the sake of tradition, rather than Modulus of elasticity, E, also known asThe behavior of a material subject to stresses is being listed as a pure number, which in fact they Young’s modulus, is the slope of this initial lin-illustrated in stress-strain curves. are. Strain is sometimes expressed as a percent ear portion of the stress-strain curve: The simplest loading to visualize is a one- elongation.dimensional tensile test, in which a uniform The shape of the stress-strain curve and val- E = S/e (Eq 3)slender test specimen is stretched along its long ues assigned to the points on the stress-straincentral axis. The stress-strain curve is a repre- curve of a metal depend on its: where S is engineering stress and e is engineer-sentation of the performance of the specimen as ing strain. Modulus of elasticity is a measurethe applied load is increased monotonically usu- • Composition of the stiffness of the material. The greater theally to fracture. • Heat treatment and conditioning modulus, the steeper the slope and the smaller Stress-strain curves in the datasheets are usu- • Prior history of plastic deformation the elastic strain resulting from the applicationally presented as “engineering” stress-strain • The strain rate of test of a given stress. Because the modulus of elas-curves, in which the original dimensions of the • Temperature ticity is needed for computing deflections ofspecimen are used in most calculations (Fig. 2, • Orientation of applied stress relative to the beams and other structural members, it is an im-Ref 13). test specimens structure portant design value. To document the tension test, an engineering • Size and shape The modulus of elasticity is determined by thestress-strain curve is constructed from the load- binding forces between atoms. Because theseelongation measurements made on the test spec- The points on the stress-strain curve that char- forces cannot be changed without changing theimen (Fig. 2). The engineering stress, S, plotted acterize the performance and are often given in basic nature of the material, the modulus of elas- ticity is one of the most structure-insensitive of the mechanical properties. Generally it is only slightly affected by alloying additions, heat treatment, or cold work. However, increasing the temperature decreases the modulus of elas- ticity. The typical modulus of elasticity of magne- sium is low in comparison with other structural metals (Table 9). Resilience is the ability of a material to absorb energy when deformed elastically and return it when unloaded. This property usually is mea- sured by the modulus of resilience, which is the strain energy per unit volume required to stress the material from zero stress to the yield stress, at B. The strain energy per unit volume for any point on the line is the area under the curve. Because of their low modulus of elasticity, magnesium alloys can absorb energy elastically. Combined with moderate strength, this provides excellent dent resistance and high damping ca- pacity. Magnesium has good fatigue resistance and performs particularly well in applicationsFig. 2 E ngineering stress-strain curve. Intersection of the dashed line with the curve determines the offset yield strength. Source: Ref 13 Introduction to Magnesium Alloys / 9

Table 9 Typical values for modulus of The current trend is to the more rational ap- rupture. Toughness is a parameter that compriseselasticity proach of basing the static design of ductile met- strength and ductility. als on the yield strength. However, because of Torsion Tests can be carried out on most ma- Elastic modulus (E) the long practice of using the tensile strength to terials to determine mechanical properties suchMetal GPa 106 psi describe the strength of materials, it has become a as modulus of elasticity in shear, shear yield familiar property; as such, it is a useful identifica- strength, ultimate shear strength, modulus ofAluminum 100 14.5 tion of a material in the same sense that the chem- rupture in shear, and ductility. The torsion testCopper 130 18.9 ical composition serves to identify a metal or can also be conducted on full-size parts (shafts,Cast iron 152 22 alloy. Furthermore, because the tensile strength is axles, and pipes) and structures (beams and easy to determine and is a reproducible property, frames) to determine their response to torsionalLead 16 2.3 it is useful for the purposes of specification and loading. In torsion testing, unlike tensile test-Magnesium 45 6.5 for quality control of a product. Extensive em- ing and compression testing, large strains canSilver 83 12 pirical correlations between tensile strength and be applied before plastic instability occurs, and properties such as hardness and fatigue strength complications due to friction between the testMild steel 211 30.6 are often useful. For brittle materials, the tensile specimen and dies do not arise.Tin 50 7.3 strength is a valid design criterion. Damping is the ability of a material to dis-Titanium 120 17.4 Measures of Ductility. Ductility is of interest sipate stain energy during mechanical vibration. in several design aspects: The energy is converted to heat. Low dampingZinc 105 15.2 materials would be selected for musical instru- • An indication of the materials formability ments where sustaining vibrations is desired.involving a large number of cycles at relatively during metalworking operations such as Low damping materials are useful where vibra-low stress. forging, rolling, wire pulling, and extrusion tion is unwanted, such as chainsaws, enclosures • To indicate the “forgiveness” of the compo- for guidance systems, and the like. Elastic limit, shown at point B in Fig. 2, is thegreatest stress the material can withstand with- nent to flow plastically before fracture should Magnesium alloys have the highest dampingout any measurable permanent strain remaining the component be subjected to forces greater index of structural metals. The specific damp-after the complete release of load. An accurate than its design criteria. ing capacity is measured at a stress of 0.1 times • To serve as a quality control measure for im- the 0.2% proof stress. In general, materials withdetermination of this elastic limit is tedious, so ayield strength value is given. purities in composition or processing high damping capacity, such as lead and cast The yield strength, shown at point YS in Fig. discrepancies. iron, have low stiffness, strength, hardness, and2, is the stress required to produce a small speci- ductility. Magnesium alloys, especially magne-fied amount of plastic deformation. The usual The conventional measures of ductility that sium-zinc K1A alloy, are the exception, havingdefinition of this property is the offset yield are obtained from the tension test are the engi- high damping capacity along with high ultimatestrength determined by the stress corresponding neering strain at fracture, ef, (usually called the strength, hardness, and ductility.to the intersection of the stress-strain curve off- elongation) and the reduction in area at fracture,set by a specified strain. In the United States, the q. Elongation and reduction in area usually areoffset is usually specified as a strain of 0.2% or expressed in percentages. Both properties are0.1% (e = 0.002 or 0.001). obtained after fracture by putting the specimen Magnesium Design for the Offset yield strength determination requires back together and taking measurements of the Long Terma specimen that has been loaded to its 0.2% final length, Lf, and final specimen cross section,offset yield strength and unloaded so that it is Af:0.2% longer than before the test. The offset yield Most of the design considerations discussed ef = (Lf − L0)/L0 (Eq 5) thus far are of a short-term nature—loading astrength is referred to in ISO Standards as the component and looking at an immediate effect.proof stress (Rp0,1 or Rp0,2). The yield strength Designers must also be concerned with the long-obtained by an offset method is commonly used term behavior of components made from mag-for design and specification purposes, because it q = (A0 − Af)/A0 (Eq 6) nesium alloys.avoids the practical difficulties of measuring the As mentioned, magnesium is reactive, so cor-elastic limit or proportional limit (Ref 10). Because an appreciable fraction of the plastic rosion is an issue that is discussed in more detail The tensile strength, or ultimate tensile deformation is concentrated in the necked region in its own section in this book.strength, Su, is the maximum load divided by of the tension specimen, the value of elongation Another issue is fatigue. Fatigue is definedthe original cross-sectional area of the specimen: depends on the gage length, L0. Therefore, the as the phenomenon leading to fracture under re- gage length is given when reporting elongation. peated or fluctuating stresses having a maximumSu = Pmax/A0 (Eq 4) Generally the test is conducted at 50 mm or 2 in. value less than the ultimate tensile strength of The reduction in area does not depend on the the material. Fatigue failure generally occurs at The tensile strength is the value most fre- specimen length. values of loads that applied statically producequently quoted from the results of a tension test. The Toughness of a material is its ability to little effect. The fatigue fracture is progressive,However, it is a value of little fundamental sig- absorb energy up to the point of fracture or rup- beginning as minute cracks that grow under thenificance with regard to the strength of a metal. ture. The ability to withstand occasional stresses action of the fluctuating stress. One means ofFor ductile metals, the tensile strength should be above the yield stress without fracturing is par- presenting fatigue behavior is in a stress-cyclesregarded as a measure of the maximum load that ticularly desirable in parts such as freight car plot (S/N diagram) such as that of commerciallya metal can withstand under the very restrictive couplings, gears, chains, and crane hooks. pure magnesium, seen in Fig. 3 (Ref 14).conditions of uniaxial loading. This value bears Toughness is a commonly used concept that is The effect of elevated temperature operationlittle relation to the useful strength of the metal difficult to precisely define. Toughness may be is addressed in the data sheets for magnesiumunder the more complex conditions of stress that considered to be the total area under the stress- alloys.usually are encountered. strain curve to the point of fracture. This area, Designers look at the entire life cycle of a For many years, it was customary to base the which is referred to as the modulus of toughness, product, from where the material comes fromstrength of structural members on the tensile UT, is the amount of work per unit volume that to its recycling and disposal. Availability is onestrength, suitably reduced by a factor of safety. can be done on the material without causing it to of the “-ilities” of concern. Magnesium is the10 / Engineering Properties of Magnesium Alloys